LOWEE SILURIAN SYSTEM. 



179 



Fig. 5. 



If the two hundred and fifty feet of vertical thickness of the 

 formation be divided into three equal horizontal parts, as 

 shown in the right hand colnmn of the diagram, almost all 

 the lead-ore will be found to be contained within the middle 

 part, together with the lower half of the upper, and the upper 

 half of the lower parts, as indicated by the brace in the dia- 

 gram. In other words, the upper half of the upper third, and 

 the lower half of the lower third of the whole thickness of 

 the formation is found to be almost entirely destitute of 

 lead-ore. This is the rule, but a few rare exceptions exist. 

 One of these exceptions was shown in the discovery of a 

 lead-bearing crevice in the extreme upper part of the Galena 

 limestone at the base of the Maquoketa shales, by Messrs. 

 McCraney and Hickok, in the summer of 1867. The value 

 and extent of the ore there has not been learned, but it is 

 supposed to be unimportant. 



This formation is in part the " Upper Magnesian Limestone' 

 of Dr. Owen. The existence of the Maquoketa shales not 

 having been recognized by him, he united, in his descriptions, 

 the Galena limestone, with the Niagara limestone, which has 

 very similar lithological characters. Thus it was erroneously 

 supposed that the Niagara limestone of New York increased 

 greatly in thickness in its westward extension, and became 

 the lead-bearing rock of the Mississippi valley. 



Fossils. Fossils are rare in this formation, as they usually 

 are in Magnesian limestones. The most characteristic species 

 is IAngula quadrata of Owen; but the genera Ortlioceras, 

 Pleurotomaria, Murcliisonia, &c, have been recognized, all 



